1. Field of the Invention
This invention relates generally to electrochemical cells and, more particularly, this invention relates to bipolar electrodes and methods of formation thereof.
2. Description of Related Art
Electrochemical cells utilizing bipolar electrode designs having reactive metal electrodes supported on substrate current collectors are well known. See, for example, Momyer et al, U.S. Pat. No. 4,269,907 (May 26, 1981), the disclosure of which is hereby incorporated by reference, wherein cells including an aqueous electrolyte, an anode of an alkali metal, such as lithium, for example, a cathode spaced from the anode, and an intercell electrical connector are disclosed. In such cells, the cathode may comprise an electrochemically active material, such as silver oxide, and the electrolyte may comprise an aqueous alkaline solution.
Momyer et al also disclose an electrochemical cell stack comprising a plurality of bipolar electrodes connected in series.
The preparation of bipolar electrodes wherein a cathode and an anode are disposed on opposite sides of an electrically conductive metallic substrate typically involves the oxidation or reduction (charging) of a precursor electrode material. For example, the preparation of a bipolar electrode with a cathode of silver oxide typically involves oxidation of elemental silver. Generally, the elemental silver of such precursor electrodes in sintered and then hot forged onto a substrate current collector. Nickel foil plated with silver, so as to facilitate adherence of elemental silver thereto, is commonly used as a substrate current collector for such bipolar electrodes.
In the oxidation of these precursor battery electrodes, the hot forgings of elemental silver are assembled into a stack in which the elemental silver and counter electrodes of nickel foil are alternated, with the elemental silver electrodes in the charging stack electrically connected in parallel to attachment to the positive post of a DC power supply. Further, all the nickel foil substrates of the counter electrodes are electrically connected in parallel for attachment to the negative post of the DC power supply. The stack is then placed into an electrolyte solution which permits the electrical contact between the precursor electrodes and the counter electrode that is necessary for the reduction or oxidation of the material of the precursor electrodes.
In principle, no precursor electrode will exhibit a voltage rise independent of the other precursor electrodes because each of the precursor electrodes is made electrically common. Thus, when one of the precursor electrodes completes oxidation prior to the others, then even an infinitesimal increase in voltage produces an increased back electromotive force (EMF) which results in a drop-off in current through the already oxidized electrode and an altering of the current path through the other electrodes and thus a different current sharing pattern in these other electrodes.
In addition, the conventional electrode formation technique of parallel oxidation is frequently accompanied by undesirable bending of the electrodes. For example, the silver oxide electrodes resulting from the use of the above-identified method of oxidation are frequently of a bent, irregular shape. This bending of the electrodes is believed to be largely a result of the stoichiometric and molar volume changes which occur during electrode formation upon oxidation of the electroactive material while the substrate is unchanged and is commonly referred to as "potato chipping".
In practice, the oxidation of silver occurs in two steps. The first step may be represented by the following equation: EQU 2Ag+2OH.sup.- .fwdarw.Ag.sub.2 O+H.sub.2 O+2e.sup.- ( 1)
Reaction (1) occurs at a standard reduction/oxidation potential of about 0.34 V which is below the voltage at which oxygen evolution will occur, i.e., the standard redox potential of oxygen is about 0.401 volt.
Once produced, Ag.sub.2 O can be oxidized to the divalent level as shown by the following equation: EQU Ag.sub.2 O+2OH.sup.- .fwdarw.2AgO+H.sub.2 O+2e.sup.- ( 2A)
Alternatively, elemental silver can be directly oxidized to the divalent level as shown by equation 2B: EQU Ag+2OH.sup.- .fwdarw.AgO+H.sub.2 O+2e.sup.- ( 2B)
Reactions (2A) and (2B) occur at potentials above 0.49 V and consequently above the redox potential of oxygen evolution. Thus, for the oxidation of silver to the divalent level, the release of oxygen at the nickel/electrolyte interface is thermodynamically favored and the efficiency of the oxidation process represented by the above equations is correspondingly decreased. Consequently, the use of this process of oxidation for bipolar electrodes generally results in electrodes having low and/or variable utilization of the active material therein and thus in electrodes of reduced capacity.
Commonly, the electrolyte used in electrode formation systems is circulated so as to minimize the likelihood of the formation of colloidal silver short circuits during the electrode formation process. The formation of colloidal silver short circuits, however, is a stochastic process and colloidal silver short circuits may be formed even when a flowing electrolyte is used.